The present invention relates to an electrolyte circulation-type metal-halogen secondary battery which can be used for example for the purpose of load leveling by storing nighttime surplus electric power and discharging stored electric power at daytime or as a power source for vehicles.
Generally, this type of known secondary battery comprises a battery body containing a positive and negative electrolytes a circulating system for circulating the electrolyte of the battery body and electrolyte storage tanks included in the circulating system.
The battery body comprises a stack of a plurality of unit secondary cells each constituting the minimum unit of the secondary battery. The unit secondary cell includes positive electrode and a negative electrode opposed at a predetermined distance apart and a separator arranged between the positive and negative electrodes so as to define positive electrode chamber between it and the positive electrode and a negative electrode chamber between it and the negative electrode. The positive electrolyte is contained in the positive electrode chamber and the negative electrolyte is contained in the negative electrode chamber.
The circulating system includes a positive electrolyte circulating system for circulating the positive electrolyte and a negative electrolyte circulating system for circulating the negative electrolyte. The positive electrolyte circulating system includes a positive electrolyte circulating pipe means arranged such that the positive electrolyte flowed out from the positive electrode chamber of each of the unit secondary cells is gathered and again returned to the positive electrode chambers and a positive electrolyte circulating pump included in the positive electrolyte circulating pipe means, and the negative electrolyte circulating system includes, similarly as the positive electrolyte circulating system, a negative electrolyte circulating pipe means arranged such that the negative electrolyte flowed out from the negative electrode chamber of each of the unit secondary cells is gathered and again returned to the negative electrode chambers and a negative electrolyte circulating pump included in the negative electrolyte circulating pipe means.
The tanks include a positive electrolyte tank for storing the positive electrolyte and a negative electrolyte tank for storing the negative electrolyte and the positive electrolyte tank is included in the positive electrolyte circulating pipe means. Also, the negative electrolyte tank is included in the negative electrolyte circulating pipe means.
The electrolyte consists of a solution of a metal-halogen compound and added to this solution is a complexing agent for converting halogen molecules to a complex compound. In addition to these, upon charge the positive electrolyte produces and contains halogen molecules and a complex compound of halogen molecules. The separator is made of an ion-permeable porous membrane which prevents the permeation of halogen molecules from the positive electrode chamber to the negative electrode chamber.
In this secondary battery, the positive electrolyte is circulated through the positive electrolyte circulating system by the positive electrolyte circulating pump and the negative electrolyte is circulated through the negative electrolyte circulating system by the negative electrolyte circulating pump. During the charging and discharging, the following oxidation-decrease reactions take place.
In other words, during the charging the metal ions in the negative electrode chamber are attracted to the negative electrode so that the metal ions are provided with electrons and reduced on the surface of the negative electrode thus depositing themselves as a metal on the surface of the negative electrode. On the other hand, the metal ions in the positive electrode chamber permeate through the separator and enter into the negative electrode chamber thereby similarly depositing themselves as a metal. In this case, the halogen ions in the positive electrode chamber are attracted to the positive electrode so that the halogen ions lose their electrons and are oxidized on the surface of the positive electrode thereby depositing themselves as halogen atoms on the surface of the anode. A halogen molecule is formed by the bonding of two halogen atoms and the resulting halogen molecules are dissolved in the positive electrolyte. Also, the halogen ions in the negative electrode chamber permeate through the separator and enter into the positive electrode chamber where similarly they are formed into halogen molecules and dissolved in the positive electrolyte. A considerable part of these halogen molecules dissolved in the positive electrolyte is converted to a halogen complex compound by the complexing agent in the electrolyte and it is then removed from the reaction system. Since the separator is made of a porous membrane which prevents the permeation of halogen molecules and the halogen complex compounds, the halogen molecules and the complex compounds remaining in the positive electrolyte are prevented from diffusing into the negative electrolyte and they are retained in the positive electrolyte.
During the discharge, the metal deposited on the surface of the negative electrode is now gradually oxidized from the surface so that it is converted to metal ions and enter into the negative electrolyte while leaving the electrons on the negative electrode.
A part of the metal ions entering into the negative electrolyte permeates through the separator and enters into the positive electrolyte of the positive electrode chamber. In this case, the halogen molecules in the positive electrolyte are provided with electrons from the positive electrode on its surface and are reduced. Thus, they are converted to halogen ions and diffused into the positive electrolyte. The halogen ions diffused into the positive electrolyte permeate through the separator and are diffused into the negative electrolyte of the negative electrode chamber. While the halogen molecules are decreased in the positive electrolyte as a result of their decrease, the complex compound of halogen molecules is decomposed due to a decrease in the concentration of the halogen molecules in the positive electrolyte and consequently the concentration of the halogen molecules in the positive electrolyte is maintained constant.
Then, it is desirable that the secondary battery used for such purposes as the storage of nighttime surplus electric power or a power source for vehicles is high in energy density or the ratio of the charged electric energy to its volume and weight in order to ensure efficient utilization of the available space for installation and efficient utilization of the stored energy. However, since the conventional secondary battery is constructed as described hereinabove, it is considerably large in volume and weight and low in energy density and therefore its use in such applications as the storage of nighttime surplus electric power or a power source for vehicles gives rise to such disadvantages that it is difficult to attain efficient utilization of the space available for installation and that it is also difficult to realize efficient utilization of the stored energy.